1. Field of the Invention
The present invention relates to an apparatus for driving an electroluminescence panel which is located on the back side of a display surface of a liquid crystal display.
2. Related Background Art
In recent years, a liquid crystal display (hereinafter, simply referred to as an LCD) has been used as a display device in word processors, personal computers, and the like.
This is because as compared with a display device of the CRT type, the apparatus can be constructed in a compact size, electric power consumption is small, and driving voltage is low, so that the LCD is suitable to realize a portable apparatus.
On the other hand, since the LCD does not emit the light by itself, there is a problem such that, for instance, it is difficult to confirm the display content under dark circumstances. Therefore, in the case of an electronic apparatus such as a word processor or the like using the LCD as a display device, it is desirable to operate the apparatus at a lighted place.
In recent years, however, there has been developed a system in which a transparent type LCD is used and an illuminating device is provided on the back side, thereby enabling the display content to be also easily confirmed even under the dark place.
In general, as such as illuminating device, an electroluminescence panel (hereinafter, simply referred to as an EL panel) is used to effectively use the characteristic of the LCD.
An AC power source is generally used to drive the EL panel. There is a feature such that the luminance of the EL panel rises with an increase in drive frequency or applied voltage of the EL panel.
On the other hand, a self-excited DC/AC inverter is ordinarily used as an apparatus to drive the EL panel. This is because since the EL lamp as a component element of the EL panel has a structure of a capacitor, a resonance circuit with its capacitive component of the capacitor and the inductive component of the selfexcited inverter is formed.
Since the capacitance of the EL lamp decreases in accordance with the lighting time thereof, the oscillating frequency of the inverter is set to automatically increase with the lighting time.
That is, since impedance of the EL lamp also increases with the lighting time, the output frequency and voltage of the self-excited inverter increase. Thus, the deterioration in luminance of an EL lamp driven by the self-excited inverter may be smaller than that driven by the commercial AC power source whose voltage and frequency are fixed.
FIG. 1 shows an example of the self-excited inverter. FIG. 2A is a curve showing the deterioration in luminance (B) to an AC power source in which a voltage (V) and a frequency (f) are fixed. FIG. 2B is a curve showing an output voltage and a frequency of the self-excited inverter and the deterioration in luminance to them.
As shown in the diagrams, it will be understood that the EL lamp using the self-excited inverter of FIG. 2B is advantageous because the luminance curve B of FIG. 2B indicative of the luminance deterioration is more gentle than that of FIG. 2A, that is, the degree of attenuation is smaller by the amount of increase in drive frequency and voltage in dependence on the lighting time of the EL lamp.
In FIGS. 2A and 2B, the time shown on an axis of abscissa corresponds to a long time and the luminance is not attenuated for a short time, e.g., a few hours.
It has been found that when the EL panel which is driven by such a self-excited DC/AC inverter is located as a back light on the back side of the LCD and used, a fringe pattern is generated on the display screen of the LCD at a certain time point, so that the display content becomes hard to see.
This is because the drive frequency f of the EL panel gradually rises due to the above-described reasons and becomes a frequency near the frequency which is integer times (harmonics) as high as a frame frequency n of the LCD, so that an interference occurs at such a time point.
On the other hand, the luminance deterioration of the EL lamp gradually progresses in dependence on the use time (on the order of hundreds or thousands hours). Therefore, there occurs a problem regarding how to suppress such a luminance deterioration. However, it is the present situation such that a technique to solve such a problem is not established yet.